1. Field of the Invention
The present invention generally relates to electrical connectors for shielded cables, and more particularly to a metallic wire braid for connecting or terminating the grounding shield of high-current power cables.
2. Description of the Prior Art
Electrical cables, such as those used for telephone lines, high voltage lines, cable television, etc., can develop faults or breaks in the line due to manufacturing defects or environmental factors. Faults occur both in underground and aerial cables. Rather than replacing an entire section of such a cable, it is expedient to expose a portion of the cable, repair or splice the fault, and place an enclosure about the splice. Splicing of electrical wires is similarly useful 20 in the placement of successive cable sections, and in cable terminations.
As shown in FIG. 1, the construction of coaxial cables, such as those used in high-voltage lines, typically includes a central conductor 1, a primary insulating layer 2, a shielded grounding layer 3, and an outer cable jacket 4. In splicing such cables, it is necessary to provide electrical continuity not only between the central conductors 1 of the two cables, but also between the sections of the grounding shields 3. One prior art connection technique for the splicing of cable shields requires the use of flat wire braids. One such prior art wire braid 5 is illustrated in FIG. 1. The braid 5 is simply wrapped around the shield 3 and held in place by adhesive tape, a constant force spring, or a metallic clamp. One end 6 of braid 5 is then connected to the shield of the other cable. Minnesota Mining & Manufacturing Co. (3M), assignee of the present invention, markets shield connectors, such as the SCOTCHLOK brand 4460 shield bond connector, which utilize flat braids (SCOTCHLOK is a trademark of 3M).
The integrity of the cable shield connection becomes particularly crucial when dealing with currents of 10,000 amperes or more. These currents may be caused by lightning surges (which may reach up to 30,000 amps), or by crossover currents in high-power cables. When such current levels are present, a poor shield connection may result in a failure of shield continuity due to excess current density. While prior art braid connectors provide a low-resistance electrical connection for cable shields (on the order of 2.5.times.10.sup.-4 ohm/foot), high current levels can nevertheless easily overload the braid. For example, the resistive power loss of a 20,000 amp current through a one foot braid surpasses 100 kilowatts. Thus, it is not surprising that reliable connections at these current levels have been extremely difficult to achieve.
One effort to alleviate this difficulty involves the provision of two separate current paths, specifically, two separate wire braids. Such a construction is used in 3M's QUICK TERM II shield termination kit (QUICK TERM is a trademark of 3M), which is depicted in FIG. 2. In this design, two braids or straps 7 and 8 are first laid along opposite sides of the cable. One end of braid 7 is fully wrapped around the first shield section 3 (i.e., at least 360.degree.). The other end of braid 7 is draped over the splice and connected to the second shield section 3' by fastening assembly 9. The second braid 8 is then wrapped over the first braid; in other words, there is no direct contact between braid 8 and shield 3. The other end of braid 8 is similarly draped over the splice and connected to shield 3' by another fastening assembly 9. The wrappings may be held in place by a constant force spring.
Theoretically, this construction would split the current between the two electrical paths, which would significantly reduce the destructive power loss. Unfortunately, however, this does not always occur in practice. For high level fault currents, the relationship of the impedance ratio of the two paths, to the current densities, is not necessarily linear. In other words, if the path through braid 8 is only slightly more resistive than that through braid 7, then much more current will flow through braid 7 than through braid 8. Of course, the path through braid 8 is more resistive, simply due to the fact that it is wrapped over braid 7, meaning that a failure in braid 7 is likely to occur, which could eventually lead to a complete failure of the shield connection.
In this regard, wrapping braid s directly against shield 3 (rather than around braid 7), does not overcome this problem, since this necessarily implies that the total length of braid 8 would be different from the total length of braid 7 and, accordingly, the impedances would also be different. This phenomenon may be intuitively understood by perceiving that, once an initial current path has been established in one of the braids, that path is preferred for the brief duration of the surge. It would, therefore, be desirable and advantageous to devise a method and apparatus which would overcome the above-mentioned limitations and minimize excess current densities across the shield connection.